3F.5 Part c

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Sebastian Lee 1L
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3F.5 Part c

Postby Sebastian Lee 1L » Mon Nov 11, 2019 1:45 pm

In Problem 3F.5 part c, it says that CHI3 has stronger intermolecular forces than CHF3 and will have a higher melting point. This difference is attributed to the difference in strength of induced dipole-induced dipole forces between the two molecules. I understand why CHI3 has stronger London forces: Iodine has a larger radius and more electrons so it is easily polarizable. High polarizability is directly related to stronger dispersion forces. However, I was wondering what role polarity and dipole-dipole forces have on the strength of attraction. Since Flourine has a much higher electronegativity than Iodine, wouldn't it have a greater dipole moment and thus create stronger dipole-dipole forces? Why is the induced dipole-induced dipole force more important when comparing the two molecules?

Hannah Lee 2F
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Re: 3F.5 Part c

Postby Hannah Lee 2F » Mon Nov 11, 2019 7:26 pm

In Problem 3F.5 part c, it says that CHI3 has stronger intermolecular forces than CHF3 and will have a higher melting point. This difference is attributed to the difference in strength of induced dipole-induced dipole forces between the two molecules. I understand why CHI3 has stronger London forces: Iodine has a larger radius and more electrons so it is easily polarizable. High polarizability is directly related to stronger dispersion forces. However, I was wondering what role polarity and dipole-dipole forces have on the strength of attraction. Since Flourine has a much higher electronegativity than Iodine, wouldn't it have a greater dipole moment and thus create stronger dipole-dipole forces? Why is the induced dipole-induced dipole force more important when comparing the two molecules?


I believe it's because both CHI3 and CHF3 have similar molecular shapes, the only difference is the I and F atoms. Though F may have higher electronegativity, the molecular shape is such that the dipole moments will "cancel out" in both CHF3 and CHI3 and make the molecules ultimately nonpolar with no significant partial charges. Because both CHI3 and CHF3 are nonpolar, the dominant intermolecular force is London forces. And since London forces are primarily determined by size/polarizability of molecule, since iodine is a bigger molecule than flourine, it will have stronger IMFs.

Sebastian Lee 1L
Posts: 134
Joined: Fri Aug 09, 2019 12:15 am
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Re: 3F.5 Part c

Postby Sebastian Lee 1L » Fri Nov 15, 2019 2:48 pm

Thank you! I talked to Dr. Lavelle and he mentioned that, like you said, the dipole moments mostly cancel out (not fully) which contributes to a relatively low polarity molecule. He also said that even though Flourine is highly electronegative, it is also pretty close to the Carbon since its atomic radius is small whereas Iodine is further from the Carbon. Since the dipole moment is proportional to the product of both charge/electronegativity and distance, the individual dipole moments aren't as disparate as I thought they were.


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